The present application is based on Japanese priority application No. 2000-267634 filed on Sep. 4, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to laser diodes and more particularly to a laser diode of lateral-mode control type formed on a GaAs substrate.
Laser diodes that use AlInP or AlGaInP for the cladding layer have various advantageous features such as laser oscillation in a visible red wavelength band, capability of focusing the laser beam to a small spot, and the like, and are used extensively for the optical source of high-density optical disk recording apparatuses including a DVD apparatus.
AlInP or AlGaInP is a material that has the largest bandgap among the III-V materials that achieve a lattice matching with a GaAs substrate and are indispensable for the material of cladding layers of a laser diode that oscillates in a red wavelength band.
FIG. 1 is a diagram showing the construction of a typical conventional ridge-type laser diode 10 having an ordinary mesa structure that forms a refractive-index waveguide.
Referring to FIG. 1, the laser diode is constructed on an n-type GaAs substrate 11 and includes a buffer layer 12 of n-type GaAs formed on the substrate 11, a cladding layer 13 of n-type AlGaInP formed on the buffer layer 12 with a composition of Al0.35Ga0.15In0.5P, and an active layer 14 of a strained multiple quantum well structure formed on the cladding layer 13.
The active layer 14 may be formed of alternate and repetitive stacking of a quantum well layer of GaInP having a thickness of 6 nm and a barrier layer of AlGaInP having a thickness of 4 nm and a composition of Al0.2Ga0.3In0.5P, wherein the foregoing stacked structure forming the active layer 14 is vertically sandwiched by a pair of optical waveguide layers of AlGaInP having a thickness of 10 nm and a composition of Al0.2Ga0.3In0.5P.
On the active layer 14, there is formed a cladding layer 15 of p-type AlGaInP having a composition represented as Al0.35Ga0.15In0.5P, and an etching stopper layer 16 of p-type GaInP is formed on the cladding layer 15. Further, another cladding layer 17 of p-type AlGaInP having a composition of Al0.35Ga0.15In0.5P and an intermediate layer 18 of p-type GaInP are formed on the etching stopper layer 16 consecutively. Thereby, the cladding layer 17 and the intermediate layer 18 are patterned by a photolithographic process to form an ordinary mesa structure constituting a ridge structure extending axially through the laser diode, and a pair of current blocking regions 19 are formed at both lateral sides of the foregoing ridge structure.
On the current blocking regions 19 thus formed, there is formed a contact layer 20 of p-type GaAs such that the contact layer 20 makes a contact with the foregoing intermediate layer 18 on the top part of the foregoing mesa region.
The ridge-type laser diode of the foregoing construction is capable of realizing laser oscillation at a desired visible wavelength by using the strained multiple quantum well structure of GaInP/AlGaInP noted before for the active layer 14. Further, the use of the current blocking regions 19 at both lateral sides of the ridge structure extending at the central part of the laser diode in the axial direction thereof enables confinement of the driving current to the foregoing ridge structure. Further, the use of the GaAs current confinement regions 19 in combination with the ridge structure is effective for confinement of the optical radiation formed in the active layer 14 in the ridge structure and for guiding therethrough.
In such a conventional ridge-type laser diode, on the other hand, it is required to conduct a photolithographic process for forming the mesa structure. Further, it is required to form the current blocking regions 19 by way of regrowth of a GaAs layer. Thus, the conventional ridge-type laser diode has a drawback of needing a complicated fabrication process. In addition, the ridge-type laser diode of FIG. 1 has a drawback of increased threshold of laser oscillation caused as a result of optical absorption by the GaAs current blocking regions 19. Thus, the conventional ridge-type laser diode has suffered from the problem of poor efficiency of laser oscillation.
It is also known to modify the ridge-type laser diode of FIG. 1 by replacing the mesa structure with an inverse-mesa structure for reducing the device resistance. However, the foregoing problems cannot be avoided by such a modification of the conventional ridge-type laser diode.
Meanwhile, the inventor of the present invention has proposed, in the Japanese Laid-Open Patent Publication 06-045708, a so-called S3 (self-aligned stepped substrate)-type laser diode 30 shown in FIG. 2.
Referring to FIG. 2, the laser diode 30 is formed on a GaAs substrate 31 of n-type, wherein the GaAs substrate 31 is formed with a stripe region of an inclined surface, which may be a (311)A surface or a (411)A surface. The substrate 31 is covered with a buffer layer 32 of n-type GaAs, wherein the buffer layer 32 forms a stripe region defined by an inclined surface in correspondence to the stripe region on the GaAs substrate 31. Further, the buffer layer 32 is covered by an intermediate layer 33 of n-type GaInP, wherein the intermediate layer 33 has an inclined stripe region formed in correspondence to the inclined stripe region on the buffer layer 32 and hence the inclined stripe region on the GaAs substrate 31.
On the intermediate layer 33, there is formed a cladding layer 34 of n-type AlGaInP in conformity with the underlying intermediate layer 33, wherein the cladding layer 34 thus formed includes an inclined stripe region in correspondence to the inclined stripe region on the intermediate layer 33.
On the cladding layer 34, there is formed an active layer 35 of a strained multiple quantum well structure similar to the active layer 14, in conformity with the underlying cladding layer 34, wherein the active layer 35 includes an inclined stripe region corresponding to the inclined stripe region formed on the cladding layer 34.
Further, a cladding layer 36 of p-type AlGaInP is formed on the active layer 35 in conformity with the underlying active layer 35, wherein the cladding layer 36 includes an inclined stripe region corresponding to the inclined stripe region formed on the active layer 35. The cladding layer 36 in turn is covered by a current confinement layer 37 of n-type AlGaInP formed in conformity with the underlying cladding layer 36, wherein the current confinement layer 37 includes an inclined stripe region corresponding to the inclined stripe region formed in the cladding layer 36.
Further, the current confinement layer 37 is covered by another cladding layer 38 of p-type AlGaInP in conformity with the current confinement layer 37, wherein the cladding layer 38 includes an inclined stripe region in correspondence to the inclined stripe region formed in the underlying current confinement layer 37. Further, the cladding layer 38 is covered with an intermediate layer 39 of p-type GaInP formed in conformity with the underlying cladding layer 38, wherein the intermediate layer 39 includes an inclined stripe region formed in correspondence to the inclined stripe region of the cladding layer 38. Further, the intermediate layer 39 is covered by a contact layer 40 of p-type GaAs formed in conformity with the underlying intermediate layer 39, wherein the contact layer 40 includes an inclined stripe region formed in correspondence to the inclined stripe region of the intermediate layer 39.
While not illustrated, the laser diode 30 of FIG. 2 further includes an n-type electrode at a bottom principal surface of the GaAs substrate 31 and a p-type electrode is formed on the contact layer 40.
The foregoing semiconductor layers 32-40 are formed consecutively on the substrate 31 thus formed with the inclined stripe region by an MOVPE process, wherein it becomes possible to dope the inclined stripe region of the current confinement layer 37 selectively to p-type and the remaining horizontal part to n-type by adding a p-type dopant such as Mg or Zn to the gaseous source during the MOVPE process of the current confinement layer 37, in addition to the n-type dopant such as Se or S.
FIG. 3 shows the efficiency of doping of various AlGaInP crystal surfaces by Mg and Zn, while FIG. 4 shows the efficiency of doping of various AlGaInP crystal surfaces by Se.
Referring to FIGS. 3 and 4, it can be seen that the efficiency of doping of Mg or Zn increases, in the AlGaInP layer, with increasing inclination angle toward the A-direction, while it can be seen also that the efficiency of doping of Se decreases with increasing inclination angle toward the A-direction.
FIG. 5 shows the relationship between the carrier concentration level (electron and hole concentration level) and the inclination angle for the AlInGaP layer that is doped simultaneously with a p-type dopant and an n-type dopant.
Referring to FIG. 5, it can be seen that there occurs a sharp decrease of electron concentration level in the AlGaInP layer when the inclination angle is increased toward the A-direction. Associated therewith, it can be seen that there occurs a sharp increase of hole concentration level with the increase of the inclination angle of the AlGaInP layer. Thus, the AlGaInP current confinement layer 37 is doped to n-type in the near-horizontal part having a surface orientation of (100) or a surface orientation near the (100) surface, while the AlGaInP current confinement layer 37 is doped to p-type in the inclined part having the (311)A or (411)A orientation.
It should be noted that such a structure can be realized also by an alternate approach such as depositing a thin p-doped layer and a thin n-doped layer alternately to form an alternate doping.
It should be noted that the foregoing S3-type laser diode 30 is already used in practice as a high-output laser diode operable in the wavelength band of 685 nm. The laser diode 30 does not require a photolithographic patterning process except for the first step of forming the inclined surface on the GaAs substrate 31, and can also eliminate the mask process and regrowth process that have been necessary in the fabrication process of the conventional ridge-type laser diode 10 when forming the current blocking regions 19. Thus, the laser diode 30 can be fabricated easily by a simple process with high yield.
By using the current confinement layer 37 thus formed, the carriers are injected selectively into the inclined stripe region of the active layer 35 and photoemission takes place in the inclined stripe region efficiently. Thereby, it should be noted that the inclined stripe region of the active layer 35 is laterally and vertically sandwiched by the AlGaInP cladding layers 34 and 36 that have a small refractive index, and there is formed an optical waveguide surrounding the inclined stripe region by the difference of the refractive index. In such a refractive-index optical waveguide, the problem of optical absorption and associated problem of increase of laser oscillation threshold that tend to appear in the case of using a complex-refractive-index optical waveguide, which uses optical absorption for the formation of the optical waveguide structure, is effectively eliminated. Further, the S3-type laser diode 30 has an advantageous feature of small astigmatism.
On the other hand, when the S3-type laser diode 30 is to be used for the optical source of recent optical disk apparatuses such as a DVD apparatus, in which a laser oscillation with a wavelength of 665 nm or less is required, it was found that there arises a problem of degradation of the characteristic temperature To in the case that the laser diode is operated at the temperature of 60-70xc2x0 C. Further, it was found that the differential efficiency of the injected current is also deteriorated, and there has been a difficulty in the conventional S3-type laser diode 30 of obtaining a large optical output. It should be noted that the characteristic temperature is an index representing the temperature dependence of the threshold current of laser oscillation. Larger the characteristic temperature To, smaller the temperature dependence of the operational characteristic of the laser diode. Thus, a laser diode having a large characteristic temperature To can operate stably without providing a particular temperature regulation.
FIG. 6 is a band diagram that represents the mechanism of the degradation of the temperature characteristic of the laser diode 30 for the state in which a bias voltage is applied to the level of causing a laser oscillation.
Referring to FIG. 6, it can be seen that the electrons injected into the n-type GaAs substrate 31 from the n-type electrode are transported along the conduction band Ec of the n-type cladding layer 34 and are accumulated in the quantum level E of the quantum well layer formed in the active layer 35. Further, the holes h injected into the p-type GaAs contact layer 40 from the p-type electrode are transported along the valence band Ev of the p-type cladding layer 36 and are accumulated in the quantum level H of the quantum well layer in the active layer 35. As a result of the recombination of the electrons and holes thus accumulated in the quantum well layer caused by the mechanism of stimulated emission, there occurs an amplification of optical radiation, which leads to the desired laser oscillation when a certain threshold of optical radiation is exceeded.
The degradation of the temperature characteristic of the laser diode is caused when the electrons e thus injected into the active layer 35 experience a thermal excitation to the level exceeding the height of the potential difference between the quantum level E of the active layer 35 and the conduction band energy Ec of the p-type cladding layer 36, wherein the foregoing barrier height can be represented as xcex94Ec+Ebilt-p, the term Ebilt-p representing the p-side component of the difference between the built-in potential and the bias voltage for the laser oscillation. The electrons thus excited cause an overflowing to the p-type cladding layer 36 without causing recombination with the holes in the active layer 35.
FIG. 7 shows the current path inside the S3-type laser diode 30 of FIG. 2, wherein those parts of FIG. 7 corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 7, it can be seen that the current confinement layer 37 includes a p-type inclined region 37A that provides a confined current path for the injected driving current. Thus, the electric current injected from the p-type electrode to the contact layer 40 is finally injected into an inclined region 35A of the active layer 35 after passing through the foregoing inclined region 37A of the current confinement layer 37 and further a corresponding inclined region 36A of the cladding layer 36 as represented in FIG. 7 by a current path I1.
Thereby, a part of the electric current thus passed through the inclined region 37A of the current confinement layer 37 may cause a lateral diffusion in the cladding layer 36 as represented in FIG. 7 by a current path I2, wherein the electric current thus diffused is injected to a horizontal part 35B of the active layer 35 via a corresponding horizontal part 36B of the cladding layer 36. Here, it is assumed that there is no overflowing of electrons explained with reference to FIG. 6 taking place in the active layer 35.
In the active layer 35, it should be noted that there exists a further current path I3 inside the active layer 35. Thus, a part of the holes injected into inclined region 35A of the active layer 35 may escape to the horizontal part 35B along the current path I3. However, such an escape of the holes from the inclined region 35A to the horizontal part 35B of the active layer 35 may be suppressed due to the increased hole concentration level in the horizontal part 35B of the active layer 35 caused by the injection of the holes to the horizontal part 35B along the current path I2 explained previously.
In the inclined region 35A of the active layer 35, there is caused a depletion of carriers as a result of the stimulated emission, and thus, there is caused a reverse flow of holes from the horizontal part 35B where the hole concentration level is large, to the inclined region 35A where the holes are depleted. From the reasons noted above, the S3-type laser diode 30 of FIG. 2 can cause laser oscillation with high efficiency even when there is caused a diffusion of electrons along the current path I2, provided that there is no overflow of electrons in the active layer 35.
When there is caused the problem of overflow of electrons explained with reference to FIG. 6, on the other hand, the holes flowing through the cladding layer 36 along the current path I2 cause a recombination with the overflowing electrons in the horizontal region 36B of the cladding layer 36, and the mechanism of suppressing the escaping of the holes from the inclined region 35A of the active layer 35 is lost. As a result, there occurs an increase of threshold of laser oscillation in the laser diode 30 and it becomes no longer possible to obtain a large optical output from the laser diode 30.
It should be noted that the problem of overflowing of the electrons appears most significantly when the laser oscillation wavelength is shifted to a short wavelength band of 665 nm band.
Accordingly, it is a general object of the present invention to provide a novel and useful laser diode and fabrication process thereof wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a so-called S3-type layer diode oscillating efficiently in the wavelength band of 665 nm or less and the fabrication process thereof.
Another object of the present invention is to provide a laser diode, comprising:
a substrate including an inclined surface region on a principal surface thereof;
an active layer formed on said substrate and including an inclined surface region corresponding to said inclined surface region of said substrate;
a first cladding layer of p-type formed on said active layer and including an inclined surface region corresponding to said inclined surface region of said active layer;
a second cladding layer formed on said first cladding layer and including an inclined surface region of p-type corresponding to said inclined surface region of said first cladding layer, said second cladding layer further including a horizontal region of n-type adjacent to said inclined surface region of p-type, said horizontal region extending parallel to said principal surface of said substrate;
a first electrode connected electrically to said substrate; and
a second electrode connected electrically to said inclined surface region of said second cladding layer,
said inclined surface region of said first cladding layer having a carrier concentration level of 1xc3x971018 cmxe2x88x923 or more,
said first cladding layer having a thickness of 0.35 xcexcm or more.
Another object of the present invention is to provide a laser diode, comprising:
a substrate including an inclined surface region on a principal surface thereof;
an active layer formed on said substrate, said active layer including an inclined surface region corresponding to said inclined surface region of said substrate;
a first cladding layer of p-type formed on said active layer, said first cladding layer including an inclined surface region corresponding to said inclined surface region of said active layer,
a second cladding layer formed on said first cladding layer, said second cladding layer including an inclined surface region of p-type corresponding to said inclined surface region of said first cladding layer, said second cladding layer further including a horizontal region of n-type adjacent to said inclined surface region of p-type, said horizontal region being parallel to said principal surface of said substrate;
a first electrode connected electrically to said substrate; and
a second electrode connected electrically to said inclined surface region of said second cladding layer,
said inclined surface region of said first cladding layer having a carrier concentration level of 1xc3x971018 cmxe2x88x923 or more,
said first cladding layer comprising an AlGaInP film containing Al with a ratio to Ga of 0.7:0.3 or more.
Another object of the present invention is to provide a laser diode, comprising:
a substrate including an inclined surface region in a principal surface thereof;
an active layer formed on said substrate, said active layer including an inclined surface region corresponding to said inclined surface region of said substrate;
a first cladding layer of p-type formed on said active layer, said active layer including an inclined surface region corresponding to said inclined surface region of said active layer;
a second cladding layer formed on said first cladding layer, said second cladding layer including a p-type inclined surface region corresponding to said inclined surface region of said first cladding layer, said second cladding layer including a horizontal region of n-type adjacent to said p-type inclined surface region, said horizontal region extending parallel to said principal surface of said substrate,
a first electrode connected electrically to said substrate; and
a second electrode connected electrically to said inclined surface region of said second cladding layer,
said inclined surface region of said first cladding layer having a carrier concentration level of 1xc3x971018cmxe2x88x923 or more,
at least a part of said first cladding layer having a bandgap larger than a bandgap of an AlGaInP film having a composition of (Al0.7Ga0.3)0.5In0.5P.
Another object of the present invention is to provide a laser diode, comprising:
a substrate having an inclined surface region on a principal surface thereof;
an active layer formed on said cladding layer, said active layer including an inclined surface region corresponding to said inclined surface region of said substrate;
a first cladding layer formed on said active layer, said first cladding layer including an inclined surface region corresponding to said inclined surface region of said active layer;
a second cladding layer formed on said first cladding layer, said second cladding layer including an inclined surface region of p-type corresponding to said inclined surface region of said first cladding layer and a pair of n-type regions adjacent to said p-type inclined surface region at both lateral sides thereof, said n-type regions extending parallel with said principal surface of said substrate;
a first electrode connected electrically to said substrate; and
a second electrode connected electrically to said inclined surface region of aid second cladding layer,
said inclined surface region of said first cladding layer having a carrier concentration level of 1xc3x971018 cmxe2x88x923 or more,
said second cladding layer having a thickness of 0.35 xcexcm or more.
Another object of the present invention is to provide a method of fabricating a laser diode, comprising the steps of:
forming an active layer on a substrate having an inclined surface region in a part of a principal surface thereof by an MOVPE process;
forming a first cladding layer of p-type on said active layer by an MOVPE process; and
forming a second cladding layer on said first cladding layer by an MOVPE process while supplying an n-type dopant and a p-type dopant simultaneously, such that said second cladding layer has p-type in an inclined surface region thereof corresponding to said inclined surface region of said substrate and such that said second cladding layer has n-type in a horizontal region thereof parallel to said principal surface.
According to the present invention, it becomes possible to increase the band discontinuity at the boundary between the p-type inclined surface region of the cladding layer and the n-type horizontal region adjacent to the p-type inclined surface region, by setting the carrier concentration level of the p-type cladding layer disposed between the active layer and the current confinement layer of a so-called S3-type laser diode to be 1xc3x971018 cmxe2x88x923 and by setting the thickness of the p-type cladding layer to be 0.35 xcexcm or more. As a result, the problem of electron overflow is suppressed effectively and the temperature characteristic of the laser diode is improved, and the laser diode of the present invention can provide a large optical output power of 50-70 mW in the wavelength band of 660 nm or less, even when the the laser diode is operated at the temperature of 60-70xc2x0 C.